Reactive sodium stearate lubricant systems are used extensively in the metal forming and metal working industry, especially in cold forming operations. Such reactive sodium stearate lubricant systems consist of high-purity sodium stearate in an aqueous solution and are often referred to simply as "reactive soaps." Such reactive soaps have been used in metal treating operations for at least the last forty years.
The reactive soaps for cold forming operations are prepared from high purity stearic acid which contains about 95 percent by weight, at a minimum, of the C-18 fatty acid and only low levels of heavy metals (i.e., generally less than about 0.01 percent by weight). Lower grades of stearic acid (often referred to as "rubber grade" stearic acid) contain less than about 70 to 80 weight percent of the C-18 chain length fatty acid. Such lower grades of stearic acid cannot be used to prepare reactive soaps. The high-purity grades of stearic acid necessary for reactive soaps are, of course, considerable more expensive than the "rubber grades." For example, stearic acid suitable for use in preparing reactive soaps currently cost about $0.55 per pound as compared to about $0.29 per pound for "rubber grade" material. Generally, a reactive soap bath contains about one pound of sodium stearate per gallon of bath. Based on the extensive use of such baths in cold forming of steel and aluminum, the cost of high purity sodium stearate represents a significant cost element in cold forming operations.
The metal work piece in cold forming operations is first phosphated in a zinc phosphate conversion bath whereby a zinc phosphate layer or coating is formed on the substrate or work piece. The phosphated work piece is then immersed in a high-purity sodium stearate or reactive soap solution (usually buffered at a pH of about 8 to 10). A reaction occurs between the sodium stearate and the zinc phosphate coating whereby a portion of the zinc phosphate coating is converted to zinc stearate via the following reaction: EQU Zn(PO.sub.4).sub.2 +6Na(OOC(CH.sub.2).sub.n CH.sub.3).fwdarw.3Zn(OOC(CH.sub.2).sub.n CH.sub.3).sub.2 +2Na.sub.3 PO.sub.4,
where n is 16. The lubricant system formed on the metal substrate consists essentially of three layers (in order): (1) a layer of zinc phosphate adjacent to the substrate surface; (2) a layer of zinc stearate formed by the above reaction on top of the zinc phosphate layer; and (3) a final or top layer of sodium stearate. In this system, the zinc phosphate is chemically bonded to the substrate surface and the zinc stearate is chemically bonded to the zinc phosphate layer.
One of the major drawbacks to the reactive lubricant system is its sensitivity to contamination by metals entering the bath during normal operation. The degree of conversion of the zinc phosphate to zinc stearate generally decreases with the age of the reactive soap bath. The decreased efficiency of a reactive soap bath is generally due to an increase in contaminants which interfere with or inhibit the reaction between the zinc phosphate and the sodium stearate. Such contaminants include heavy metals (e.g., zinc and iron) which may be introduced through "drag-in" from earlier stages of the treatment process and from reactions with the substrate, as well as calcium and magnesium salts from the aqueous media. Such metal contaminants form their corresponding stearates in the bath. Generally, when the combined metal contaminant levels in the reactive soap exceed about 0.1 percent by weight, the lubricant system is no longer satisfactory for most cold forming operations and is considered "spent."
The most common method of treating a spent reactive sodium stearate bath is to simply discard the bath and prepare a new bath using fresh, high-purity sodium stearate. Normally, the spent reactive soaps are hauled away by a waste disposal company for treatment and disposal. Such a system is economically and environmentally unsound. As environmental regulations on the disposal of such wastes increase, these costs are expected to increase. Most conventional methods for removing the metal contaminants from spent reactive soap solutions cannot be used. For example, any filtering media or system would be quickly clogged with fatty acids or soaps if such filtering methods were attempted. To date the only alternative to direct disposal of the spent reactive soaps is the mechanical removal of the metal contaminants by periodic or continuous centrifuging operations. Such centrifugation methods are messy, difficult, and costly and have not, therefore, been widely accepted. Although the metal working and metal forming industry has been aware of these problems and has been forced to deal with these problems on a daily basis for almost forty years, no generally satisfactory solution has been found.
It is desirable, therefore, to provide a method by which spent reactive soap lubricants can be easily treated. It is also desirable to provide a method by which high purity stearic acid can be recovered from spent reactive sodium stearate lubricant baths. It is also desirable to provide a method by which the useful lifetime of reactive sodium stearate solutions can be extended. The present invention achieves these and other objectives as fully described in this specification using a straight-forward and simple treatment process.